Production of pulverized cellulose



Patented July 3, 1934 UNITED STATES PRODUCTION OF PULVERIZED CEILUIDSE Milton 0. Schur and Benjamin G. Hoes, Berlin, N. H., assignors to Brown Company, Berlin,

N. H., a corporation of Maine No Drawing. Application September a, 1931, Serial No. 561,001

9Claims.

In the manufacture of certain ultimate products, it is desirable to use powdered cellulose as a filling or loading material. Thus, powdered cellulose may be advantageously used in the loading or filling of papers, fabrics, or the like, together with, if desired, a sizing agent. In combination with plastics, e. g., urea-formaldehyde or phenolic condensation products, the powdered cellulose furnishes body, resiliency, opacity, and machining qualities. In many of these connections, however, it is most important that the powdered cellulose be stable, i. e., maintain its colorlessness against the influences of light, moisture, air, and heat. I

There are types of cellulose fibers which stand up well against atmospheric influences and against the action of heat without undergoing discoloration. For instance, powdered cotton fiber or powdered refined wood pulp of high alpha cellulose content are characterized by-their permanency of color even when combined with plastics which are moulded into articles of the desired shape at elevated temperature. The powdering of cellulose fiber is, however, a performance which calls for the expenditure of enormous quantities of energy when practiced in such instrumentalities as ball mills. A large capital investment is required for such powdered equipment, which must be of large size for a given output of powdered product. Great care must constantly be exercised to avoid contamination of the powdered fiber by the material of construction of the powdering equipment.

We have discovered that a refined wood pulp may be transformed chemically to a pulverulent condition in which it possesses remarkable stability, even against the action of heat, despite the fact that considerable of its alpha cellulose content may be transformed into other forms of cellulose. The usual pulps of commerce, e. g., bleached sulphite or bleached soda pulp, are lacking in stability, to begin with, and this instability persists or is aggravated through a chemical treatment which lowers the alpha cellulose content of such fiber. We have discovered that a refined wood pulp of high alpha cellulose,

content is stable not only to begin with, but retains such stability even though its alpha cellulose content is materially reduced more especial- 1y by sufiiciently drastic hydrolyzing treatment with various acid solutions to cause partial or complete pulverization. By the characterization refined wood pulps" herein given, we mean pulps, such as sulphite or kraft, which have undergone special processing, including digestion in alkaline liquors, designed to increase their alpha cellulose content to at least about 93% and to whiten them. Such refined wood pulps are thus much freer from non-alpha cellulose components, including ligneous, resinous, and

coloring matter, beta and gamma celluloses, and the like, than the unrefined wood pulp. Consequently when subjected to acid-hydrolyzing treatment designed to embrittle such pulp and to convert it to a partially or completely pulverized condition, there is evidently insufiicient color-generating substance formed from nonalpha cellulose components to give rise to dis coloring tendencies, even though considerable alpha cellulose has been converted into other formsof cellulose. These altered celluloses are, from a practical point of view, as stable in the atmosphere and under the action of heat as the original alpha cellulose from which they were formed.

In accordance with our invention, therefore, we subject previously purified wood pulps of an alpha cellulose content of at least about 93% to the action of hydrolyzing aqueous liquors under conditions designed to pulverize the pulp fibers partially or completely. The hydrolyzing liquors are preferably of acid character, as acids function to embrittle the fiber in a highly satisfactory manner, especially when they are strongly ionized in water, as is true of mineral acids like sulphuric, hydrochloric, etc. A wide variety of time, temperature, and pressure conditions may be used in the acid solution treatment of the purified pulp. If the conditions chosen are such as only partially to pulverize the fiber, the pulverization may be completed in a ball mill or other suitable grinder, which, however, completes pulverization with the consumption of little mechanical energy on account of the friable state of the fiber. y

We shall now recite illustrative procedures falling within the purview of our invention. Thick sheets of refined wood pulp, known as drier sheets, may be used as the raw, material. The refined wood pulpmay, for example, have an alpha cellulose content of about 94.5% and be previously purified sulphite pulp. The sheets may be immersed in a large excess of sulphuric acid solution inthe ratio of, say, 1 part of fiber to 20 parts of acid solution containing, say, 5% sulphuric acid, based on liquid. The solution may I then be heated, as with live steam, to the boiling point under atmospheric pressure, which may be maintained for a period of, say, six hours. During the boiling operation, the sheets tend to disintegrate and yield a suspension of fine fibers, which may be allowed to settle. The supernatant acid solution may be decanted from the layer of cellulose, and the cellulosic slurry then washed free of acid solution with water. The washed mass may then be pressed free of easily removable water, and the resulting cake dried as in a heated chamber. The cake may then be subjected to a light rubbing action, as in a smallsized cofiee mill, wherewith large quantities may easily be converted into free-flowing powder with small power consumption. This powder is useful as a filler in products of the class hereinbefore mentioned.

Another example of procedure may be about as follows. Refined wood pulp of alpha cellulose content of 93%, prepared from a sulphate or kraft fiber base, may be pumped in aqueous slush form into a digester provided with an acid-proof lining, wherein it may be heated as, say, a 5% suspension in a 2% sulphuric acid solution, untilthe pressure in the digester is 25 pounds gage per square inch. This pressure may be maintained for one hour, whereupon the contents ofthe digester may be blown into a receiver, from which the slurry may be pumped to a filter press or into a centrifuge and wrung free of excess solution. The mass may then be subjected to a washing operation and pressed into cakes, which may either be dried and reduced to free-flowing powder, as already described, or which may be charged directly into the beater, if it is to be used for papermaking purposes, without an intermediate drying. A powdered product'so prepared without mechanical treatment passes practically entirely through a 40-mesh screen, but it may be made to consist of particles of finer mesh by longer boiling in the acid solution, or through the use of stronger acid solution, higher temperature, mechanical means, etc.

In the examples cited, the alpha cellulose content of the pulp dropped from the original values to about '75%,-a value much lower than the alpha cellulose present in the usual unrefined commercial wood pulps, like sulphite, soda, kraft, etc. Yet, as already indicated, this powder of low alpha cellulose content is more stable and much more useful than the mechanically powdered, unrefined wood pulp. The acid hydrolyzing treatment is thus accompanied by the embrittlement and pulverization of the fiber and a sharp decrease in alpha cellulose content. It has long been known that the acid treatment of cellulose fibers makes for embrittlement of the fiber. Contrary to what was to be expected, however, the embrittlement of purified wood pulp by acid hydrolysis, even though accompanied by great loss in alpha cellulose, is not attended by impartation to the cellulose of instability, which militates against the use of the powdered product in papermaking and in plastics moulded even at temperatures ranging from 125 to 180 C.

The digestion of unrefined or ordinary chemical wood pulps, such as sulphite, in alkaline liquors to remove non-alpha cellulose components from the pulp and thereby to produce a pulp of higher alpha cellulose content, say one of an alpha cellulose content of at least about 93% has now been known for some time. The step of digesting such refined wood pulps in acid-hydrolyzing liquors, for instance, in dilute mineral acid solutions, more than undoes the work of refinement effected by the alkaline refining liquors, but, contrary to expectation, the degradation products of the alpha cellulose are of a difierent and more stable sort, particularly as regards color, than are the non-alpha cellulose components associated with the unrefined wood pulp. It is for this reason that despite the fact that we wind up with a chemically powdered product whose alpha cellulose content may be much lower than that of the unrefined wood pulp, nevertheless our product is of greater value than the mechanically powdered-substantially undegraded unrefined wood pulp.

We claim:

1. A process which comprises refining preliberraw material but whose color stability is greater than that of the mechanically pulverized unrefined cellulose pulp.

2. A process which comprises refining chemical wood pulp in an alkaline liquor to an alpha cellulose content of at least about 93%, and then hydrolyzing such refined pulp under conditions to degrade it to a powder whose alpha cellulose content is lower than that of the unrefined wood pulp used as raw material but whose color stability is greater than that of the mechanically pulverized unrefined chemical wood pulp.

3. A process which comprises chemically liberating cellulose pulp from raw cellulosic material and purifying it in an alkaline liquor to an alpha cellulose content of at least about 93%, and then digesting such purified pulp in an acid hydrolyzing liquor under conditions to degrade it to a powder whose alpha cellulose content is as low as about but which has higher color stability than mechanically pulverized sulphite wood pulp.

4. A process which comprises first dissolving in an alkaline liquor non-alpha cellulose components, including ligneous matter and resins, from cellulose fiber, thereby increasing its alpha cellulose content, and then hydrolyzing such fiber under conditions to degrade it into a powder of an alpha cellulose content lower than that of the cellulose fiber used as starting material.

5. A process which comprises extracting in an alkaline liquor non-alpha cellulose components, including ligneous matter and resins from chemical wood pulp, thereby increasing its alpha cellulose content, and then subjecting the pulp to the action of an acid-hydrolyzing liquor under conditions to degrade it into a powder of an alpha cellulose content lower than that of the chemical wood pulp used as starting material.

6. A process which comprises refining wood pulp in an alkaline liquor to an alpha cellulose content of at least about 93%, and then digesting such refined pulp in a mineral acid solution under conditions to degrade it into a powder of an alpha cellulose content lower than that of the wood pulp used as starting material.

7. A process which comprises refining wood pulp in an alkaline liquor to an alpha cellulose content of at least about 93%. and then digesting such refined pulp in a sulphuric acid solution under conditions to degrade it into a powder of an alpha cellulose content lower than that of the wood pulp used as starting material.

8. A degraded, white, acid-pulverized cellulose pulp derived from raw cellulosic material purified to an alpha cellulose content of at least about 93%, said degraded, cellulose powder having an alpha cellulose content even less than that of an unpurified chemical wood pulp but having greater color stability than the unpurified chemical wood pulp.

9. A color stable white powder derived from pulp of at least about 93% alpha cellulose content by an acid treatment which lowers the alpha cellulose content in the powder.

MILTON O. SCHUR. BENJAMIN G. HOOS. 

